1. Field of the Invention
The present invention relates to an evaporative emission control device, and more specifically, to absorption control of a fuel evaporative gas escaping from a closed fuel tank by using a canister.
2. Description of the Related Art
A conventional technology of preventing a fuel evaporative gas evaporated within a fuel tank from escaping into atmosphere provides a canister connected to a fuel tank, and a fuel tank shutoff valve (sealing valve) that is controlled to close the fuel tank except during fuel supply, which is interposed in a path connecting the fuel tank to the canister, opens the sealing valve during fuel supply to cause the fuel evaporative gas to flow out towards the canister, and thus absorbs the fuel evaporative gas by using the canister.
However, if external temperature rises while the fuel tank is being closed by the sealing valve, fuel in the fuel tank is evaporated, and the pressure in the fuel tank is increased and turned into high pressure.
In the above situation, to prevent the evaporative gas from escaping into atmosphere along with fuel supply, when a fuel supply operation is detected, the sealing valve is opened, and a fuel supply port is inhibited from being opened until the pressure in the fuel tank is sufficiently reduced.
However, the reduction of the pressure in the fuel tank is a long process, which means that it takes a long time before fuel supply starts.
Given this factor, there has been developed a technology in which, if the pressure in the fuel tank is increased during the operation of the engine and the purge process, the sealing valve is opened to release the fuel evaporative gas in the fuel tank into the intake passage of the engine without absorbing the fuel evaporative gas in the canister, thereby reducing the pressure in the fuel tank (Japanese Patent No. 4110932).
The evaporative fuel processor described in the above publication conducts an opening/closing control on a purge vacuum switching valve (purge control valve) for opening/closing a connecting passage that introduces the fuel evaporative gas into the intake passage and the sealing valve at the same time during the operation of the engine in order to reduce the pressure in the fuel tank. In this manner, the purge control valve and the closing valve operate in consort. The fuel evaporative gas that is released into the intake passage of the engine through the connecting passage passes through the canister, so that the fuel evaporative gas is partially absorbed by the canister. This generates the possibility that the amount of the fuel evaporative gas that the canister is capable of absorbing during fuel supply is decreased.
To solve this problem, the canister is disposed in the connecting passage extending between the sealing valve and the purge control valve with a canister shutoff valve (vapor solenoid valve) interposed in the connecting passage. If the pressure in the fuel tank during engine operation is increased and has to be reduced, the vapor solenoid valve is closed, and the sealing valve and the purge control valve are alternately opened. This way, the fuel evaporative gas is prevented from being absorbed by the canister while the fuel evaporative gas in the fuel tank is being released into the intake passage. Some evaporative fuel processors close the purge control valve and open the sealing valve and the purge control valve during fuel supply, to thereby absorb the fuel evaporative gas in the fuel tank by using the canister.
However, if the sealing valve and the purge control valve are alternately opened and closed during engine operation, negative pressure is generated in the connecting passage due to the intake negative pressure of the engine. In addition, the pressure in the fuel tank is positive as the result of fuel evaporation, so that there causes a great pressure difference between in front of and behind the sealing valve. If the sealing valve is opened to release the pressure in the fuel tank under the above situation, there is the possibility that the fuel in the fuel tank is sucked out into the connecting passage due to the great pressure difference or that a valve located in a fuel cutoff valve for preventing a fuel leakage from the fuel tank, which is disposed inside the fuel tank, is attached to the connecting passage. Such fuel suction into the connecting passage incurs a malfunction in engine operation, and may cause a pressure rise in the fuel tank, leading to damage to the fuel tank.